This invention relates to xerography and more particularly relates to a method for enhancing or controlling the functional electrical life and stability of copolymers of butadiene and terminally unsaturated hydrocarbon nitriles used in xerographic devices.
In the conventional office copier systems, many of the devices and subsystems are made of polymer materials where conductivity control and relaxation behavior (electrical) is important. These polymer materials are also generally characterized by moisture insensitivity, mechanical durability and systems stability. Exemplary of such devices and subsystems include (1) transfer belts, webs and scrolls; (2) photoconductor coatings and overlays; (3) development devices such as donor plates; and (4) durable mechanical devices with electrical stability such as, biased cleaning blades, cleaning brushes, webs or rolls, paper handling devices and static controllers in processor sorters, transport belts and the like. The poly(butadiene/terminally unsaturated hydrocarbon nitrile) copolymer compositions and methods for enhancing or controlling the functional electrical life and stability of these copolymers may be used in these devices and subsystems, and a detailed description is given herein of the utility of these compositions and methods when used to make a biasable transfer member.
In conventional xerography a photosensitive plate, which consists of a photoconductive coating placed over a conductive backing, is charged uniformly and the charge plate is then exposed to a light image of an original. Under the influence of the light image the charge on the plate is selectively dissipated to record the original input information on the plate in the form of a latent electrostatic image. The latent image is developed, or made visible, by applying oppositely charged toner particles to the plate surface in a manner so that the toner particles are attracted into the imaged areas. The developed images are generally transferred from the photoconductor to a final support material, such as paper or the like, and affixed thereto to form a permanent record of the original.
Image transfer from the photoconductor to the final support material may be accomplished by means of a corona induction using a corona generator, or it may be accomplished by a roller or belt electrode biased to a certain potential, such electrode being referred to as a bias transfer member (roll or bolt). The corotron system is relatively simple, but the charges deposited by the corotron electrostatically tack the final support material, such as paper, to the original toner support, such as, the photoconductor, in addition to creating the desired electric field affecting transfer of the toner to the paper. The strong attraction between the paper and the original toner support makes it mechanically difficult to separate or detack the two supports.
Transfer of developed images from the photoconductor to the final support material with the aid of a biased transfer member is now well known in the art, and such a member generally avoids severe tacking problems which are encountered when the corona induction system is utilized. A bias transfer roll is disclosed by Fitch in U.S. Pat. No. 2,807,233 where a metal roll coated with a resilient coating having a resistivity of about 10.sup.6 to 10.sup.8 ohm cm is used as a bias transfer member. Shelffo in U.S. Pat. No. 3,520,604 suggests that in order to create the proper environment for the duplicating mode, a transfer roll is used and is made of a conductive rubber having a resistivity in the range of from about 10.sup.11 to about 10.sup.16 ohm cm. A bias transfer member, that is, a member for electrically cooperating with a conductive support surface to attract electrically, charged particles from the support surface towards the member, is described by Dolcimascolo et al in U.S. Pat. No. 3,702,482. In Dolcimascolo et al, the bias transfer member has a conductive substrate for supporting a bias potential thereon, an intermediate blanket of, for example, polyurethane rubber, placed in contact with the substrate having an electrical resistivity capable of readily transmitting the bias potential on the substrate to the outer periphery of the blanket and a relatively thin outer coating of, for example, polyurethane, placed over the blanket having an electrical resistivity to minimize ionization of the atmosphere when the transfer member is placed in electrical cooperation with the image support surface and provides a good toner release property enabling the device to be cleaned of the toner.
Other bias transfer members are described by Eddy et al in U.S. Pat. No. 3,959,573 incorporated herein by reference where there is described and claimed biasable transfer members having a coating of a hydrophobic elastomeric polyurethane and having a resistivity in which the change in resistivity is substantially insensitive to changes in relative humidity. Exemplary of the polyurethanes therein, is a polyurethane made by reacting 83.7 percent (by weight) butadiene-acrylonitrile copolymer with 16.2 percent diisocyanate in the presence of a catalyst, the copolymer comprising about 85 percent butadiene and about 15 percent acrylonitrile. Disclosed therein is the use of ionic additives for reducing the resistivity of the hydrophobic elastomeric polyurethane. Examples of the ionic additives include quaternary ammonium compounds exemplary of which is tetraheptyl ammonium compounds. Seanor et al in U.S. Pat. No. 3,959,574, incorporated herein by reference, describe and claim biasable transfer members comprising a conductive substrate and at least one coating of an elastomeric polyurethane having an additive therein for controlling the resistivity of the polyurethane, the coating being placed over the conductive substrate. Exemplary of the additives therein which provide a method and composition for controlling the resistivity of biasable transfer members, are the quaternary ammonium compounds, exemplary of which are tetraheptyl ammonium bromide, trimethyloctadecyl ammonium chloride and benzyltrimethyl ammonium chloride.
Although the foregoing references provide polyurethane materials which have many desirable electrical and stability characteristics, it is desirable to improve the electrical life of the materials used in such devices and subsystems. It is also important to control the conductivity or electrical relaxation behavior (ionic mobility versus equilibrium rate between ionized and un-ionized salt so that new ions are provided as electrolysis depletes existing ions) of the polymers used in the foregoing devices where concurrent demands for moisture insensitivity, mechanical durability and systems stability are also important. By electrical life is meant controlled (constant) resistivity with time under an applied electrical field. The functional life of a component, such as a bias transfer roll, is directly related to maintenance of this constant controlled resistivity region. The copolymerization of butadiene and a terminally unsaturated hydrocarbon nitrile which selectively introduces nitrile groups into the polymer, greatly enhances the ionization of ionic additives, e.g., quaternary ammonium salts resulting in the need for lower salt molalities and as shown in our copending application, results in improved materials stability and solubility. However, this increases ionic mobility and therefore results in a more rapid variation in the resitivity vs. time of the material.
The functional life of the devices using the butadiene copolymerized with terminally unsaturated hydrocarbon nitriles, such as acrylonitrile, of the present invention, for example, bias transfer devices, is largely determined by the stability of the output current and or voltage versus time. Bias roll power supplies are generally constant current or constant voltage output devices with upper current or voltage limits which respond to changes in the resistivity of the bias roll material. Changes in the resistivity of the base material versus time are reflected in voltage demands required to maintain the constant current output of the material of which the device is made. In addition, it is necessary that the additive or additives used to control the V versus I (or resistivity) relationship remain soluble and uniform throughout the transfer material. If additive insolubility occurs, negative xerographic responses such as image washout, field non-uniformities and decreased environmental latitude are known to occur.
Specifically, it has been shown that a highly polar polymer network such as 15 percent acrylonitrile (ACN)/butadiene copolymer with a symmetrical quaternary additive such as tetraheptylammonium bromide (THAB) affords enhanced specified ionization of the additive and therefore improved additive efficiency. This is evidenced by the reduced additive concentration required to attain specific resistivity levels. This higher degree of ionization enables the additive to remain soluble but simultaneously results in increased ionic mobility and therefore a more rapid variation in ionic depletion and hence more rapid variation in base roll resistivity. By way of example, the butadiene homopolymer (i.e., no ACN content) requires high levels of quaternary additive (ca. 2 percent) to attain a specific resistivity due to low additive ionization. This low degree of ionization leads to solubility deficiencies of the resistivity control agents in the low polarity matrix with the resultant xerographic problems as previously described. Concurrently, however, this low polarity network results in decreased ion mobilities and hence extended functional electrical life of the material.